Method of producing planographic printing plates

ABSTRACT

A PHOTOGRAPHIC PRINTING PLATE IS PREPARED FROM A SHEET MATERIAL CARRYING AN EXTERIOR HYDROPHILIC COLLOID LAYER HAVING SUBSTANTIALLY CONCENTRATED AS ITS FREE SURFACE A PATTERN OF FINELY DIVIDED SILVER PARTICLES, BY CONTACTING THE SAME WITH AN AQUEOUS ALKALINE LIQUID FIXER COMPOSITION CONTAINING AN OXIDIZING AGENT FOR SAID SILVER, PARTICLES, E.G. CYANOFERRATE (III)-IONS, AN ORGANIC THIOXO COMPOUND OR TAUTOMER THEREOF FOR CONVERTING THE PATTERN OF SILVER PARTICLES INTO A HYDROPHOBIC INK-RECEPTIVE PATTERN, A HIGHER FATTY ACID, E.G., OLEIC ACID, FOR IMPROVING THE INK-RECEPTIVITY OF SAID PATTERN, AND A WATER-SOLUBLE NITRITE, SUCH AS ALKALI METAL NITRITE OR AN AMMONIUM NITRITE, FOR STABILIZING THE DISPERSION OF THE HIGHER FATTY ACID IN THE FIXED LIQUID.

United States Patent 3,676,125 METHOD OF PRODUCING PLANOGRAPHIC PRINTINGPLATES Louis Maria de Haes, Edegem, Belgium, assignor to Agfa-Gevaert,Mortsel, Belgium No Drawing. Filed Apr. 24, 1970, Ser. No. 31,787 Claimspriority, application Great Britain, Apr. 25, 1969, 21,279/ 69 Int. Cl.G03f 7/02 US. CI. 96-33 11 Claims ABSTRACT OF THE DISCLOSURE Aphotographic printing plate is prepared from a sheet material carryingan exterior hydrophilic colloid layer having substantially concentratedat its free surface a pattern of finely divided silver particles, bycontacting the same with an aqueous alkaline liquid fixer compositioncontaining an oxidizing agent for said silver particles, e.g.cyanoferrate(HI)-ions, an organic thioxo compound or tautomer thereoffor converting the pattern of silver particles into a hydrophobicink-receptive pattern, a higher fatty acid, e.g., oleic acid, forimproving the ink-receptivity of said pattern, and a water-solublenitrite, such as alkali metal nitrite or an ammonium nitrite, forstabilizing the dispersion of the higher fatty acid in the fixed liquid.

The present invention relates to a method of producing a planographicprinting plate by converting a silver image into a hydrophobicink-receptive pattern.

The preparation of a printing plate according to the present inventioncomprises treating a sheet material having an outer hydrophilic colloidlayer carrying at its surface a pattern of finely divided silverparticles, with an aqueous fixer comprising an oxidizing agent for saidsilver particles, an organic compound that converts the pattern ofsilver particles in a hydrophobic ink-receptive pattern, a higher fattyacid having from 12 to 22 carbon atoms, and a water-soluble nitrite, thesaid outer hydrophilic colloid layer having been hardened before, duringor after said treatment at least to such an extent that no substantialamounts of colloid are transfered during printing.

The present invention also relates to an aqueous fixer as describedabove.

The presence of the higher fatty acid in the aqueous fixer results in animprovement of the hydrophobic inkreceptive characteristics of theprinting portions. This effeet is especially pronounced when the sheetmaterial to be treated with the fixer for the production of the printingplate is a photographic material comprising at its surface a silverpattern, which has been formed according to the silver complex diffusiontransfer process from silver complexes diffusing image-wise distributedfrom an underlying image-wise exposed silver halide emulsion layer.Consequently, the method of the present invention is particularly suitedfor application in connection with this specific embodiment. However,the higher fatty acid does not remain homogeneously distributed in thefixer and easily tends to unmix during aging. This tendency is fullyovercome by the addition of the water-soluble nitrite. Thus, an aqueousfixer having excellent keeping qualities is obtained.

If a water-soluble cyanoferrate(III) is used as the oxidizing agent inthe fixer, the water-soluble nitrite exerts a further favourable effectin preventing an undesirable yellow, brownish, and even bluish colouringof the fixer during aging. Although such a colour change has nosubstantial effect on the very fixing properties, it might confuse theoperator as to the nature as well as to the quality of the liquid.

ice

All water-soluble nitrites, i.e. all organic and inorganic compoundsreleasing nitrite ions in an aqueous medium are suited for use accordingto the present invention. Preferably simple water-soluble inorganicnitrites are used, e.g. sodium nitrite, potassium nitrite, or ammoniumnitrite. The nitrites are generally applied in an amount comprisedbetween 2 and 20 g. per litre of fixer.

The higher fatty acids to be incorporated into the fixer includesaturated as well as unsaturated acids, e.g. lauric acid, myristic acid,palmitic acid, stearic acid, and oleic acid. The unsaturated higherfatty acids and especially oleic acid have proved to be the most suitedones for the purpose of the present invention. These higher fatty acidsmay be incorporated into the fixer from a solution in a water-miscibleorganic solvent, e.g. a lower aliphatic alcohol such as propanol, or inthe form of a water-soluble salt (soap), e.g. the sodium or potassiumsalt. A watersoluble salt of the higher fatty acid may also be producedin situ by the presence of suitable alkaline substances such astrisodium phosphate or sodium hydroxide, in the fixer. Thus, theexpression higher fatty acids also includes the water-soluble saltsthereof. The higher fatty acids are preferably applied in an amountcomprised between 5 and g. per litre of fixer.

The oxidizing agent present in the fixer liquid preferably is awater-soluble cyanoferratefl'll), and the fixer is then preferablybuffered at a pH value between 7 and 12.

Suitable organic compounds for converting the pattern of silverparticles into a hydrophobic ink-receptive pattern are thioxo compounds,including the tautomeric structures thereof.

For more details on suitable oxidizing agents, specific organiccompounds for converting the pattern of silver particles intoink-receptive parts and other usual ingredients of the fixer, and forgeneral data about the production of the silver pattern (preferablyaccording to the silver complex diffusion transfer process) as well asabout the preparation and the use of the printing plate reference ismade to the U8. patent specification 3,083,097, and US. Pat. 3,582,328issued June 1, 1970, corresponding to the published Dutch patentapplication 68/ 00431, and to the U.S. patent applications Ser. Nos.887,323, 887,324 and 887,387, now abandoned. The sheet material carryingthe silver pattern at its surface (e.g. produced from silver complexes,which have diffused image-Wise from an underlying silver halide emulsionlayer) preferably comprises a paper support or ahydrophobic syntheticfilm support including a paper sheet provided with a thin layer of ahydrophobic synthetic polymeric material, e.g. polyethylene.

The essential disclosure of the three applications identified above isreproduced below.

The preparation of printing plates according to the subject matter ofsuch applications comprises treating a sheet material having an outerhydrophilic colloid layer carrying at its surface a pattern of finelydivided silver particles, which pattern has been produced in aphotographic way by the application of the silver complex diffusiontransfer process being carried out with a single photographic materialcomprising both the silver halide emulsion layer and the substance orsubstances for promoting the silver deposition from the diffusingcomplexed silver halide, i.e., the so-called development nuclei, with anaqueous fixer composition containing an oxidizing agent for the silverparticles and an organic thioxo compound or tautomer thereof thatconverts the pattern of silver particles to a hydrophobic ink-receptivepattern, said outer hydrophilic colloid layer being hardened before,during or after the said treatment, and at least to such an extent thatno substantial amount of colloid is transferred on printing.

The silver complex diffusion transfer process is especially suited forproducing the pattern of silver particles,

sinoe according to this process the silver deposition mainly occurs atthe outer surface of the layer provided with the substance or substancesfor promoting the silver deposition from the diffusing complexed silverhalide, i.e., the so-called development nuclei.

According to a first diffusion transfer embodiment, the developmentnuclei can be supplied to the light-sensitive material by the processingliquid for carrying out the diffusion transfer process. In this case itis sufficient to dispose of a light-sensitive material only, which isprovided either or not with a colloid layer on top of the silver halideemulsion layer. The development nuclei will deposit from the processingliquid onto the surface of the light-sensitive material at thedevelopment stage and as a result the diffusion transfer silverdeposition Will also occur at this outer surface of the light-sensitivematerial. It is the latter diffusion transfer silver image that will beconverted into the ink-receptive printing areas of a planographicprinting plate and not the silver image resulting from the primarydevelopment of the image-wise exposed silver halide.

According to the embodiment wherein the development nuclei are suppliedby the processing liquid and preferably according to said embodimentwherein also a colloid layer is provided on top of the silver halideemulsion layer it is also possible to transfer a diffusion transfersilver image containing stratum to a suitable support and to use theresulting material as starting material for the preparation of aplanographic printing plate according to the method of the invention. Inconnection with this method for the preparation of diffusion transferimages, more particularly the embodiment, wherein a colloid layer isprovided on top of the silver halide emulsion layer, there can bereferred to the United States patent specification 3,335,005.

As a variant of the said first diffusion transfer embodiment thedevelopment nuclei for the diffusion transfer image formation, insteadof being supplied by the alkaline processing liquid can also be suppliedto the light-sensitive material by a special treatment with a separateliquid com position containing such nuclei. This treatment may be thelast step in the preparation of the material or a treatment that iscarried out after the exposure of the lightsensitive material and beforewetting with the alkaline processing liquid for carrying out thediffusion transfer image formation.

According to a further diffusion transfer embodiment the developmentnuclei may be provided in a colloid layer on top of the silver halideemulsion layer at the preparation stage; in this way a multilayermaterial is obtained. Such a multilayer material is known among othersfrom the United States patent specifications 2,725,298 and 2,843,485 andfrom the United Kingdom patent specification 758,627. After thediffusion transfer image formation, the said colloid top layercontaining the diffusion transfer silver image is transferred to anothersheet thus providing a colloid layer having on its free surface thepattern of finely divided silver particles as required for thepreparation of a planographic printing plate according to the method ofthe invention. This transfer to another sheet may be avoided bysubstantially or completely concentrating the development nuclei at thefree surface of the outer colloid layer of the multilayer material,whereby the diffusion transfer silver deposition substantially takesplace at this outer free surface. In this connection there may bereferred to the French patent specification 1,569,398.

The development nuclei, instead of being present in the said colloidlayer on top of the silver halide emulsion layer, may also be providedbetween the light-sensitive emulsion layer and the said colloid layer.After the diffusion transfer image formation the said colloid layer istransferred to another sheet material and carries the diffusion transfersilver image on its free surface.

It is also possible to use a multilayer material comprising alight-sensitive silver halide emulsion layer on top of animage-receiving layer containing development nuclei.

Such a multilayer material is described i.a. in United Kingdom patentspecification 654,631 and to the United States patent specifications3,020,155, 3,203,796 and 3,325,284. .After the diffusion transfer imageformation the lightsensitive emulsion layer may be removed by detachingin an aqueous rinsing bath as a coherent membrane or in flakes, bydissolving or by stripping off either or not by means of an auxiliarysheet, whereupon the colloid imagereceiving layer with the silverpattern formed on its surface becomes visible.

In order to obtain at the surface of the outer hydrophilic colloid layera diffusion transfer silver deposition as high as possible thedevelopment nuclei are very often substantially or completelyconcentrated at the imagereceiving side of that layer. This can be veryadvantageously effected by the application of the development nucleithrough an after-treatment of the colloid layer with a liquidcomposition containing such nuclei. As already mentioned above thisliquid composition may be the processing liquid itself for carrying outthe diffusion transfer image formation or another aqueous dispersion orsolution of said nuclei, which is provided on top of the hydrophiliccolloid layer before the diffusion transfer image formation takes place.In order to keep the development nuclei in dispersion a minor amount ofa hydrophilic protective colloid is often added to the dispersion. Thisamount of colloid does not suffice to form a layer and in most cases ishardened at the hardening stage of the hydrophilic colloid layer or bydiffusion of hardener from said layer.

The above is a non-limitative list of embodiments for the production ofthe silver pattern at the surface of an outer hydrophilic colloid layer.The sheet material carrying the silver pattern generally has a rathersimple composition and comprises a suitable support such as a usualhydrophobic flexible film support, e.g. a sheet of cellulose triacetateor of polyethylene terephthalate, or a paper sheet provided eitherdirectly or indirectly, e.g., by means of a suitable subbing layer, witha hydrophilic colloid outer layer having at its surface a pattern offinely divided silver particles. Often still a silver halide emulsionlayer is present between the colloid outer layer and the support andantihalation dyes or pigments are provided.

According to a preferred embodiment, a photographic material is usedwhich in order comprises a support sheet, preferably a paper support ora hydrophobic film support, an antihalation layer, a silver halideemulsion layer and development nuclei on top thereof. If theantihalation layer is gray the correctness of the exposure can becontrolled by judging the diffusion transfer image in transmittancebefore carrying out the fixing step. In order to obtain a diffusiontransfer silver deposition that will be sufficiently high, the saidphotographic material has to be kept for a short while, e.g. for about10 seconds, in the dark after it has been wetted with the alkalineprocessing liquid for carrying out the diffusion transfer imageformation. The application of the fixer composition may already occurduring the period the multilayer material is kept in the dark. However,this application of fixer may also occur thereafter.

For the production of the diffusion transfer silver image a photographicmaterial comprising development nuclei on top of the silver halideemulsion layer may be provided in roller form. After image-wiseexposure, e.g. an episcopic exposure in a camera or an exposure througha trans parent original in contact with the photographic material, thelatter may be guided automatically through a usual processing unitcontaining the alkaline processing liquid and comprising guiding anddriving means as generally known in the art. In order to overcomeprocessing irregularities during the cutting stage of the multilayermaterial, an empty tray with loose roller, which is lifted by thematerial during cutting, may be provided between the exposure unit andthe processing unit. Another compact processing unit for such aphotographic material, in-

cluding the exposure station is described in the French patentspecification 1,582,876.

In producing the pattern of silver particles according to the diffusiontransfer process at least part of the compounds that are essential oruseful for carrying out said process may be provided in the sheetmaterial(s) used. Among these compounds there may be mentioneddeveloping agents, preservatives for these developing agents, complexingagents, stabilizers, alkaline substances, blacktoning agents, hardeners,including latent hardeners, and softening agents. By incorporating thegreater part or all of these compounds in the sheet material(s) used theaqueous processing liquid can be reduced to an aqueous solution ofalkaline substances and even to pure water.

Suitable substances for promoting the silver deposition from thediffusing complexed silver halide are sulphides of heavy metals such asthe sulphides of antimony, bismuth, cadmium, cobalt, lead, nickel,silver and zinc. Other suitable salts are the selenides, polysulphides,polyselenides, mercaptans and thin(II) halides. Heavy metals or theirsalts and fogged silver halides are suitable too. The complexed salts oflead and zinc sulphides are active alone as well as mixed withthioacetamide, dithiobiuret, and dithiooxamide. Heavy metals, preferablysilver, gold, platina, palladium, and mercury may be used in theircolloidal form. From these metals the noble metals are the most activeones.

The support of the photographic material comprising the silver halideemulsion layer may be any usual flexible support sheet e.g. a papersheet or a transparent hydrophobic usual film support, such as a supportof cellulose triacetate or of a polyester e.-g. polyethyleneterephthalate.

The exposure latitude of the photographic material in most cases can beincreased considerably and also the image sharpness of the silverpattern and as a consequence thereof, the sharpness of the final resultof printing can be improved by applying antihalation dyes or pigments.

These dyes or pigments may be present in the silver halide emulsionlayer or in the support, but preferably are incorporated into a layersituated between the silver halide emulsion layer and the support. If atransparent support is used the antihalation dyes or pigments may beapplied at the rear side of the material or on top of the emulsion layerdependent on the manner in which the exposure is carried out, viz at thefront side or through the support. Preferably a red or blackantihalation dye or pigment is used.

Any silver halide emulsion of the negative or direct positive type maybe used dependent on the nature of the original to be reproduced.Preferably, however, emulsions of the negative type having a rather highsensitivity of the order as normally required for camera exposure areused. As a matter of fact, although the exposure of the silver halideemulsion layer may be carried out according to any usual technique e.g.in contact, reflectographically, by transmission or episcopically,mostly an episcopic exposure in a camera is carried out, particularlywhen the development nuclei are provided on top of the silver halideemulsion layer as detailedly described hereinbefore. The silver halideemulsion layer generally comprises an amount of silver halide equivalentto form about 0.5 g. to about 1.5 g. of silver nitrate and preferablyamounting only to the equivalent of about 1 g. of silver nitrate. Thismeans a considerable enconomy of silver halide with respect to thesilver halide content of emulsion layers commonly used in the productionof diffusion transfer copies. If necessary a suitable subbing layer isprovided for strongly adhering the hydrophilic colloid layer(s) to thesupport sheet.

For the preparation of the hydrophilic colloid outer layer anyhardenable hydrophilic colloid may be used. Suitable hydrophiliccolloids include gelatin polyvinyl alcohol, casein,carboxymethylcellulose and sodium alginate. Of course, the nature of thehardener used is dependent on the type of hydrophilic colloid to behardened.

According to a preferred embodiment of the present invention thehydrophilic colloid binder of said outer layer comprises a mixture of aproteinaceous hydrophilic colloid, preferably gelatin, and a hydrophiliccolloid that is a latent hardener for said proteinaceous colloid, i.e.that in the alkaline medium wherein the production of the diffusiontransfer image takes place hardens said proteinaceous colloid. Suitablehydrophilic colloids of this type are addition products of highmolecular weight hydroxyl group-containing compounds With acrolein,described in the United States patent specification 3,382,077 andespecially water-soluble hydroxy alkyl esters of alginic acid,particularly propylene glycol esters of algininc acid, described in theUnited States patent specification 3,378,373. If gelatin is used in thehydrophilic colloid outer layer, it may be submitted to a treatment asdescribed in the United States patent specification 3,089,770 in orderto improve the printing characteristics of the planographic printingplate.

Hardening of the hydrophilic colloid binder of the outer layer may occurbefore, during or after the treatment with the fixer composition andmust occur at least to such an extent that no substantial amount ofcolloid is transferred on printing neither to the rollers for applyingwater and ink to the printing plate nor to the material to be printed.In other words hardening i.e. insolubilizing in water and strengtheningagainst mechanical damage must occur at least to such an extent that thematerial obtained can be used as planographic printing plate.

If the said hardening is carried out before the treatment with fixer, itmay occur by adding generally known hardening agents for the hydrophiliccolloid(s) of the outer layer such as formaldehyde, glyoxal, mucochloricacid and chrome alum to the coating composition of the outer layer, atthe surface of which the silver pattern will be produced, and/ or to thecoating composition of another layer, with which the said outer layer isin water-permeable relationship whereby hardening of the said outerlayer takes place by diffusion of hardener from said other layer to saidouter layer. When, as already referred to above, a coating compositioncomprising development nuclei, either or not together with a minoramount of a hydrophilic colloid to keep the development nuclei indispersion is applied on top of the said outer layer the hardeners canbe also be incorporated into said coating composition.

Hardening of the hydrophilic colloid binder of the outer layer may alsooccur during the production of the silver pattern according to thesilver complex diffusion transfer process. Said hardening may occur byincorporatmg hardeners in at least one of the processing liquids and/ orby incorporating latent hardeners, as already referred to above, intothe photographic material to be hardened.

Finally, hardening of the outer hydrophilic colloid layer can also occurafter the production of the silver pattern namely by treatment with ahardening liquid. This liquid may be an aqueous hardening compositionapplied before the treatment with the fixer, the fixer compositionitself or an aqueous hardening composition applied after the treatmentwith the fixer.

The presence of certain hydrophilic colloid binders e.g.carboxymethylcellulose, gum arabic, sodium alginate, propyleneglycolester of alginic acid, hydroxyethyl starch, dextrine,hydroxyethylcellulose, polyvinylpyrrolidone, polystyrene sulphonic acidand polyvinyl alcohol in the outer hydrophilic colloid layer carrying atits surface the pattern of silver particles often improves thehydrophilic, ink-repellent properties of the non-printing areas of theprinting plate finally obtained. Also hygroscopic substances likewisecalled hydrophilic softening agents, e.g. sorbitol, glycerol,trihydroxyethyl ether of glycerol and turkish red oil, and certainwetting agents, i.e. those mentioned hereinafter incorporated in atleast one hydrophilic colloid layer of the sheet material carrying thesilver image at its surface will improve the hydrophilic properties ofthe non-printing areas.

The hydrophilic colloid outer layer also may advantageously comprise aconsiderable amount of pigment particles that Will prevent the so-calledscumming (i.e. inkacceptance that arises in the non-printing areas ofthe printing plate after a certain number of copies has been printed).The usual inorganic pigments e.g. barium sulphate, titanium dioxide,china clay and silica applied from a colloidal solution, have proved tobe particularly suitable for this purpose. The pigment particles aregenerally homogeneously applied in such an amount that about -20 g.thereof are present per sq. m. of the hydrophilic colloid outer layer. Asimilar anti-scumming etfect may also be obtained by adding at least onemember selected of colloidal silica, an inorganic acid e.g. o-phosphoricacid, a hydrophilic softening agent as those mentioned above and asuitable wetting agent to the fountain solution used during the printingprocess. Suitable wetting agents are i.a.

sodium dodecylsulphate sodium tetradecylsulphate RCH -SO Na wherein Rrepresents an alkyl group comprising from 14 to 18 carbon atoms o sCCH2(CHz)15 Ha 1l NaO s l and R =hexadeeyl 90% octadecyl oetadecenyl 4%A very suitable fountain solution comprises water, glycerol,o-phosphoric acid and colloidal silica.

For the production of the planographic printing plate, the sheetmaterial comprising an outer hydrophilic colloid layer having at itssurface a pattern of finely divided silver particles is treated with anaqueous fixed composition as described above.

As disclosed in US. Pat. 3,083,097, various oxidizing agents areavailable for oxidizing the finely divided silver particles in thesurface pattern including the Water-soluble cyanoferrate(III) compoundsmentioned above. Other oxidizing agents include Water-soluble Cr salts,e.g., sodium or potassium dichromate (application S.N. 887,- 323),Water-soluble iron(III) salts, e.g., iron(III) chloride (applicationSer. No. 887,324), and Water-soluble tin(IV) salts, e.g., tin(IV)chloride, water-soluble cerium(III) salts, e.g., cerium(III) sulfate,and water-soluble persulfates, e.g., potassium persulfate (applicationSer.No. 887,387). Where the oxidizing agent is selected from the latergroups, the pH of the composition is desirably made acidic by means of asuitable acidic compound e.g., acetic acid or boric acid. A pH of notmore than 4.0 is preferable for iron(III) salts and less than 3.5 ispreferably for the Cr salts. The oxidizing agent mostly is incorporatedinto the fixer composition in a concentration of from 10 to 300 g. perliter. In the case of CR salts, the presence of halide ions often has afavorable effect on the action of the fixer composition. So,water-soluble chlorides, bromides or iodides, such as potassium bromide,are often incorporated into the fixer.

The oxidizing agent used in carrying out the present method may be usedin combination with at least one other oxidizing agent, e.g., a lead(II)ion-releasing compound,

N 1120 c=s 1120-1511 imidazolidine-4-on-2-thiones including substitutedderivatlVeS Such as @iJm phenyltetrazoline-thiones including substitutedderivatives such as I'q s=o \N Hs't 1i tetrahydro pyrimidine-Z-thionesincluding substituted derivatives such as tetrahydroanddihydro-oxazine-Z-thiones and thiazine- 2-thiones including substitutedderivatives such as 1,3,4-dihydro thiadiaZine Z-thioneS includingsubstituted derivatives such as thiourea compounds including substitutedderivatives preferably N-phenyl thiourea compounds such as (11) CH I? acompound of another type and that also converts the sllver image partsin hydrophobic ink-receptive parts is (12) IIICS The compounds thatconvert the silver image parts in hydrophobic ink-receptive parts may beadded directly to the fixer, but are often added from an aqueoussolution or from a solution in a suitable organic, water-misciblesolvent such as ethanol, isopropanol, ethylene glycolmonomethyl etherand dirnethylformamide. They must be at least partially soluble in thefixer composition. They are mostly applied in an amount of from 0.5 to 5g. per litre fixer. It is remarkable that most of the suitable compoundsfor converting the silver image parts in ink-receptive parts aregenerally known black-toning agents in the silver complex diffusiontransfer process.

An explanation of the reaction mechanism according to which the silverimage parts are converted into hydrophobic ink-receptive parts cannot begiven with certainty. Indeed, it might be a chemical transformation thattakes place or more likely a physical adsorption to the silver imageparts. In any event the fact remains that at least a superficialoxidation of the silver pattern is necessary. With certain thioxocompounds it is possible that oxidation products of these compounds arepresent in the fixer and it is not beyond possibility that preciselythese oxidation products are responsible for the hydrophobizing effectobtained in carrying out the method of the invention.

The fixer may comprise some further compounds for improving thehydrophilic ink-repellent properties of the non-printing areas. Suitablesuch compounds are colloidal silica and hydrophilic colloid binders,hydrophilic softening agents, inorganic acids and wetting agents alreadymentioned and exemplified above. These compounds mostly are added to thefixer in a concentration ranging from about 5 to about 200 g. per litre.

The fixer may also contain compounds for improving the ink-receptivecharacter of the printing areas such as higher fatty acids, e.g. oleicacid, which often are applied together with a water-soluble nitrite inorder to keep them homogeneously distributed in the fixer, andhydrophobic softening agents, e.g. dimethyl phthalate, dibutylphthalate, the phthalic acid ester with the following formula:

triphenyl phosphate, and tricresyl phosphate. The hydrophobic softeningagents mostly are added to the fixer in an amount of from about 5 toabout g. per litre. It is also possible to apply at least one of thesecompounds in about the same concentrations after the treatment with thefixer composition.

The fixer composition used in carrying out the method of the presentinvention is very stable to aerial oxidation and to temperaturefluctuations and it may be used for the production of many printingplates without having to frequently adjust the processing time. Manyprinting plates (about 100 plates of 21 cm. x 29.7 cm. size per litre offixer) can be produced before the fixer composition is fully exhausted.

At the moment the treatment with the aqueous fixer starts the outercolloid layer showing the silver pattern may be in dry as well as in Wetcondition. A superficial oxidation of the silver pattern sufiices;generally, however, a more penetrating oxidation is advantageous.Generally the treatment with the fixer does not last long, mostly notlonger than about 20 seconds and can be accelerated by increasing theconcentration of the components in the fixer. The plate may be storedfor a long time before being fixed and even thereafter it may be storedfor a long time before being used in the printing process. Preferably,however, the fixing step is carried out just before printing.

The fixer as well as the development or activating liquid for theproduction of the difiusion transfer image can be applied in differentways, e.g. by spraying, by rubbing, with a lick roller, or by dippingthe material to be treated in the liquid composition. Also the fixingstep of the printing plate may occur automatically by conducting theplate through a device having a narrow channel filled with the fixercomposition and at the end thereof between two squeezing rollers. Inorder to reduce the risk for aerial oxidation of the fixer to a minimum,funnel-shaped inand outlet openings may be provided at the ends of thenarrow channel and an upperlying plastic bag containing the fixer liquidmay be used. If the apparatus is not used, the bag is moved downwardsand the fixer runs back from the channel into the bag.

The production of a silver image on top of the outer hydrophilic colloidlayer (occasionally including the image-wise exposure of the silverhalide emulsion layer) and the treatment with the fixer may occurautomatically, e.g. in processing stations as described above andoccasionally may be carried out both in a compact processing unitcomprising both processing stations.

After the application of the fixer the sheet material is ready for useas a printing plate. This means that no further treatment with a lacquercomposition for strengthening the printing parts is necessary.

This is an important advantage of the method of the present inventionover most of the conventional methods hitherto used for the productionof a planographic printing plate that require the application of alacquer as an essential feature.

Nevertheless, in some cases the hydrophobic character of theink-receptive parts may still be improved and their mechanical strengthmay be reinforced by applying a lacquer thereon. Suitable lacquercompositions are solutions of oils, waxes, and resins in organicsolvents. Suitable organic solvents are cyclohexanone, acetone, butanol,monomethyl ether of ethylene glycol, monoethyl ether of diethyleneglycol, tetrahydrothiophene-1,1 dioxide, diacetone, dioxane,1,2-dichloroethane, ethyl acetate, trichloroethylene, butyl butyrate,diethanolamine and dimethylformamide. Mixtures of such organic solutionswith an aqueous phase or dispersions of such organic solutions in anaqueous-phase are suitable too. In that case the aqueous phase maycontain thickeners or other usual compounds for improving thehydrophilic character of the non-printing areas of the lithographicprinting plate as described above. Suitable lacquers are described inthe United Kingdom patent specifications 967,598,

968,706 and 1,004,342, and in the United States patent specification3,378,372. Resins that have proved to be especially suitable forimproving the ink-receptive character of the printing areas andstrengthening them are phenol-formaldehyde resins e.g. the usualphenol-formaldehyde resins, o-cresol formaldehyde resins and p-tert.-butylphenol formaldehyde resins, alkyd resins e.g. rosin maleic acidesters, epoxy resins e.g. condensation products of a poly(aryl ethyleneoxide) with an acid anhydride, an amine or another suitable compound andepoxidized polyesters.

The resin, wax or oil used mostly is present in a concentration of fromabout 100 to about 500 g. per litre of liquid lacquer composition. Theliquid lacquer composition may be applied after the aqueous compositioncontaining the oxidizing agent has been applied and while the printingplate is still wet. Only when an emulsion lacquer is used the plate neednot necessarily be wet at the stage the lacquer is applied. The lacquermay be applied by dipping, spraying, spreading or by means of a materialsoaked therewith. The lacquer is rubbed out e.g. with a plug of wadding.The hydrophobic solid substances settle on the image areas and improvethe hydrophobic ink-receptive character thereof. At the same time themechanical strength of the printing areas is improved. The adherence ofthe lacquer to the printing areas may still be improved by heating theplate.

Instead of being applied by means of a separate aftertreatment of theprinting plate, the liquid lacquer composition may also be mixed withthe aqueous fixer composition. The ratio of the organic phase thatcontains the hydrophobic solid substances to the aqueous phase thenmostly is comprised between 1:1 and 1:10.

The printing plate has to be wet at the stage the fatty printing ink isapplied. This is generally known in the art and it is usual to apply anaqueous liquid before applying the printing ink. This may occur by meansof a wet sponge or by means of the fountain arrangement of the printingmachine. This wetting has to be more intensive as the printing plate isdrier, e.g., when it has been stored for a long time after its fixationand occasional lacquering.

The following examples illustrate the method and the fixer of thepresent invention.

EXAMPLE 1 A thin grey antihalation layer was applied to an extra strongpaper support of 135 g. per sq. m. at a ratio of 1 litre per 40 sq. m.with a dispersion of 3 g. of lamp black in l litre,,of.a ,4% aqueoussolution of gelatin.

To this: antihalatiqn layer a common high-sensitive silverchlorobromidegelatin emulsion layer comprising hydroquinone and1-phenyl-3-pyrazolidinone was applied in such a way that per sq. m. werepresent: an amount of silver'halide equivalent to 1 g. of silvernitrate, 1 g. of hydroquinone, and 0.5 g. of l-phenyl-3-pyrazolidinone.

The following coating composition was applied to the dry silver halidegelatin emulsion layer in a ratio of 100 g. per sq. m.:

Water926 ml.

Gelatin-10 g.

12.5% aqueous saponin8 ml. 20% aqueous formaldehyde-56 ml.

Then the following liquid was applied at a rate of 22 g. per sq. m.:

Water-890 ml.

12.5 aqueous saponinlO ml.

Aqueous dispersion of colloidal nickel sulphide nuclei comprising per100 ml. 0.2 g. of nickel sulphide and 10 g. of gelatin-4 ml.

The light-sensitive multilayer material thus obtained was image-wiseexposed to an original.

12 After the image-wise exposure the multilayer material was guidedthrough a processing unit containing the following alkaline liquid:

Water-1000 ml.

Sodium phosphate-12-water75 g. Anhydrous sodium sulphite40 g. Potassiumbromide-0.5 g.

Anhydrous sodium thiosulphate-IO g.

Subsequently, the multilayer material was kept for approximately -l0seconds in the dark to make possible an intense silver deposition bydiffusion transfer. A sharp bronzed legible diffusion transfer imagebecame visible on a gray background.

The plate was then rubbed for approximately 20 seconds with thefollowing fixer:

Water-50 ml.

Trisodium phosphate-12-water4.0 g.

Potassium cyanoferrate(II[)--9..8 g.

Ethylene glycol monoacetate5 ml.

n-Propanol20 ml.

Oleic acid-3.0 ml.

Sodium nitrite-0.5 g.

10% solution of 1-allyl-imidazolidine-2-thione in ethylene glycolmonomethyl ether-5.2 ml.

Distilled water up to ml.

The pH of this buffered fixer composition was 7.6.

A high-quality planographic printing plate having positive image valuesin respect of the original was obtained.

The fixer described above has excellent keeping qualities. The colour ofthe fixer does not change during aging and the oleic acid remainshomogeneously distributed throughout the fixer. A similar fixercontaining, however, no sodium nitrite becomes brownish and bluish afterabout one week, and the oleic acid unmixes from the aqueous phase. Saidunmixing is of course highly unwanted since it impairs the fixingproperties.

EXAMPLE 2 Example 1 was repeated with the diflerence, however, thattrisodium phosphate-lZ-Water was omitted from the fixer and oleic acidwas replaced by 3 g. of the sodium salt of oleic acid. The fixer thusobtained has a pH-value of 8.8 and shows good keeping properties whichare similar to those of the fixer described in Example 1.

By proceeding in this way a high-quality printing plate was obtained.

I claim:

1. Aqueous fixer liquid composition for use in the productionof aplanographic printing plate from a sheet support having an outerhydrophilic colloid layer carrying substantially concentrated at itsfree surface a pattern of finely divided silver particles, said fixercomprising an oxidizing agent for said silver particles, an organicthioxo compound or tautomer thereof adapted to convert the pattern ofsilver particles into a hydrophobic ink-receptive pattern, a higherfatty acid having about 12-22 carbons in the fatty chain dispersed insaid aqueous liquid in an amount sufficient to improve the hydrophobiccharacteristics of said ink-receptive pattern, and a water-solublenitrite in sufiicient amount to significantly improve the stability ofthe fatty acid dispersion.

2. Aqueous fixer composition according to claim 1, wherein the fattyacid is an unsaturated higher fatty acid.

3. Aqueous fixer composition according to claim 1, wherein the higherfatty acid is present in an amount of between 5 and g. per litre offixer composition.

4. Aqueous fixer composition according to claim 1, wherein thewater-soluble nitrite is present in an amount of between 2 and 20 g. perlitre of fixer composition.

5. Aqueous fixer composition according to claim 1, wherein the oxidizingagent is a water-soluble cyanoferrate(II:I) and the fixer composition isbuffered at a pH value between 7 and 12.

6. In the method of producing planographic printing plates whichcomprises contacting a sheet support having an exterior hydrophiliccolloid layer carrying substantially concentrated at its free surface apattern of finely divided silver particles, with an aqueous alkalinefixer composition containing an oxidizing agent for said silverparticles, and an organic thioxo compounds or a tautomer thereof adaptedto convert the pattern of the oxidized silver particles into a.hydrophobic ink-receptive pattern, the improvement of incorporating insaid fixer composition an amount of dispersed higher fatty acid havingabout 12-22 carbons in the fatty chain thereof sufiicient to improve thehydrophobic characteristics of the ink-receptive pattern together with asufficient amount of a water-soluble nitrite to significantly improvethe stability of the dispersion of said fatty acid.

7. A method according to claim 6, wherein the higher fatty acid ispresent in an amount of between about 5-120 g., and the water-solublenitrite in an amount of about 2-20 g. per liter in the fixer compositionare used and said solution is buffered at a pH value of about 7-12.

8. A method according to claim 6, wherein the pattern of silverparticles is produced according to the silver complex diffusion transferprocess.

9. A method according to claim 8, wherein the silver pattern has beenformed from image-wise distributed silver complexes diffusing from anunderlying image-wise exposed silver halide emulsion layer.

10. A method according to claim 6, wherein the sheet support is a papersupport, hydrophobic synthetic film sup port, or a paper sheet providedwith a thin layer of a hydrophobic synthetic polymeric material.

11. A method according to claim 6, wherein the higher fatty acid is anunsaturated higher fatty acid selected from the group consisting oflauric acid, myristic acid, palmitic acid.

References Cited UNITED STATES PATENTS 3/1963 Lassig et a1 9629 L 7/1966 Wagemans et a1 9629 L

